a) Field of the Invention
This invention relates to pigment dispersions for the production of color filters (hereinafter called "pigment dispersions for color filters"), which are excellent in fluidity, storage stability, dispersion stability, and the like.
b) Description of the Related Art
A color filter, which is useful for the production of a liquid crystal color display, an image pickup device or the like, has conventionally been produced primarily by spin-coating a color filter substrate with a pigment dispersion for a color filter--said pigment dispersion containing pigments of three colors, that is, red (R), green (G) and blue (B) dispersed in a solution of a photosensitive resin--to form a color film, exposing the color film to light through a photomask, and then developing the exposed color film to form the color film into a pattern so that desired pixels are formed on the color filter substrate.
As primary pigments for use in the production of color filters, phthalocyanine green, for example, C.I. pigment green (hereinafter called "P.G.") 36 is generally used as a green color, anthraquinone red, for example, C.I. pigment red (hereinafter called "P.R.") 177 is generally employed as a red color, and phthalocyanine blue, for example, C.I. pigment blue (hereinafter called "P.B.") 15:6 is generally used as a blue color. There are however differences between the hues of these pigments and color characteristics required for liquid crystal displays. For a green pigment and a red pigment, a yellow pigment is therefore used in combination in a small amount as a complementary color, and for a blue pigment, a purple pigment is also used in combination in a small amount as a complementary color.
A pigment dispersion for a color filter is generally composed of a pigment, a dispersant, a film-forming resin and a liquid medium. As the film-forming resin, an acrylic resin having an acid value high enough to permit development with an aqueous alkaline solution after the formation of a film is mainly adopted. However, a pigment dispersion formed of the above-described conventional pigment and the acrylic resin of the high acid value has poor storage stability in many instances. Flocculation of the pigment therefore takes place so that the viscosity of the pigment dispersion tends to become higher with time.
When a color filter is produced by using the pigment dispersion accompanied by such difficulties as described above, the pigment dispersion is coated on a color filter substrate by spin-coating. If the viscosity of the pigment dispersion is high or if pigment particles in the dispersion undergo flocculation and the pigment dispersion exhibits thixotropic viscosity, The pigment dispersion so coated becomes thicker around a central portion of the substrate. This leads to occurrence of unevenness in color hue and a difference in color density between the resulting color film at the central part of the substrate and that at a peripheral part of the substrate upon production of a large-screen color filter.
Accordingly, a pigment dispersion for a color filter must be in such a stable dispersion state of the pigment as being free from mutual flocculation of its pigment particles and must have a low viscosity of from 5 to 20 centipoises viscosity and excellent storage stability, despite its pigment content is generally in a range of from 5 to 10 wt. %. With a view to meeting performance requirements for such pigment dispersions for color filters, a variety of dispersants are disclosed in JP 60-237403 A and JP 60-247603 A. Pigment dispersions making use of these pigment dispersants (hereinafter simply called "dispersants") are accompanied by a drawback in that they show neither sufficiently low viscosity nor sufficiently high dispersion stability and also by another drawback in that the maximum absorption wavelengths at RGB pixels in color filters produced from these pigment dispersions are caused to shift toward a lower wavelength side or a shorter wavelength side, thereby lowering their color quality as color filters for liquid crystal displays.